1. Field of the Invention
The invention relates generally to a ranging and detecting system in which a coherent beam of light is transmitted, reflected from a target, received and processed to extract information concerning the target and in which passive radiation emitted by the target is also detected.
More particularly, the invention relates to such a system in which the coherent beam of light is scanned through a predetermined pattern by a rotating wedge. The system may be of the doppler detection type in which the frequency of the received signal beam is compared with a portion of the transmitted signal beam to determine the velocity of targets.
2. Description of the Prior Art
Systems were developed in the past for detecting only one of the passive or active radiation emitted from a target. In the passive systems, infrared radiation inherently emitted by a target in accordance with well-known radiation phenomena was detected and processed. The size, type, and temperature of the target could be determined from the processed signals. In the active systems, the target was illuminated with coherent radiation from a scanning laser source. A portion of the illuminating radiation was reflected back to the source by a target. Position and velocity of the target could be determined from the processed returned signals. No known systems of the prior art combined both active and passive detection and processing capabilities in a unitary system.
In scanning systems of the present type, the received signal beams are processed by mixing them with a portion of the transmitted signal beam termed the local oscillator signal. Because of the finite transit time of the transmitted beam to the target and back, the received signal beams strike the rotating wedge at a different angle than that at which they were transmitted. This change in angle between transmitted and received beams causes the position of the received beam as it is focused upon the detector to vary. The position upon the detector varies with both the range to the target and the scanning rate. Because both of these parameters may be varied during a system scanning operation, fixed compensation cannot be used.
One prior solution was simply to slow down the rate of rotation of the scanning wedge to a speed such that the local oscillator signal and received signals were in sufficient alignment to produce a processable signal. Unfortunately, the maximum permissible scanning rate under such circumstances is too slow to be of use in most practical surveillance and reconnaissance applications.